1. Technical Field
The present disclosure relates to detection of ionization current in spark plugs of internal combustion engines and more particularly to a method of sensing ionization current and to a spark plug structure allowing an extended sensing during the different phases of an engine cycle.
2. Discussion of the Related Art
In internal-combustion engines, the fuel-air gaseous mixture ignition is activated by a HV electric discharge across the spark plug electrodes. During combustion, molecules in the combustion chamber ionize and a ionization current flows in the electrodes of the spark plug.
The classic structure of a spark plug is shown in FIG. 1.
Commonly, spark plugs have a metal casing 1 fitted over an insulator 2, usually molded or cast around a first metal electrode 3, axially extending from a high voltage terminal 4 at the outer end of the insulator to the inner tip 5 from which the first electrode 3 protrudes. The insulator 2 isolates the first electrode 3 from the threaded metal casing 1, permanently fitted over the isolator, adapted to mount the spark plug into a threaded hole of a cylinder head of the engine, and from a ground electrode 6, integral to or shorted to the metal casing 1 and having an end extension that is bent toward the tip of the first electrode 3, protruding out of the inner end of the insulator in order to form a discharge gap 7.
When the spark plug is mounted, the threaded metal casing 1 is driven tight into the threaded hole of a cylinder head of the engine and thus the ground electrode 6 is shorted to the electrical systems ground node. A high voltage generated by the ignition coil (supplied thought an Electronic Ignition Controller circuitry), is applied to the terminal of the first electrode 1. A spark then occurs in the discharge gap 7, the air/fuel mixture in the combustion chamber is ignited and a ionization current flows in the electrodes.
It is known in the art that the ionization current may be processed to provide early detection of plug fouling, for example, and more generally to monitor the combustion process. In particular, the sensed ionization current may be used in control loops to adjust ignition timing, valve timing, fueling, and/or airflow, for example, to better manage the combustion process.
FIG. 2 shows an exemplary graph of ionization current versus crank angle, in which three main phases of the combustion process are highlighted, namely the ignition phase, the flame-front phase and the post-flame phase. The ionization current is characterized by high frequency oscillations during the ignition phase (generating high frequency spectral components), then by a rapid increase and decrease during the flame-front phase and finally by a slow increase and decrease during the post-flame phase.
Ionic currents in spark plugs may be measured at the secondary of the ignition coil, substantially by connecting a sensing circuit between the low-voltage terminal of the secondary winding of the ignition coil and ground, as schematically shown in FIG. 3. Such sensing circuits are used to monitor the ionization current after the combustion has occurred.
It would be desirable a technique of sensing ionization currents crossing the electrodes of a spark plug during all phases of the combustion process.